At present, the pixel part of a liquid crystal panel forms images through switching of thin film transistors fabricated with an amorphous or polycrystal silicon film on a substrate of glass or synthesized quartz. In the case that it becomes possible to simultaneously form a driver circuit (at present mainly installed independently outside of the panel) to drive pixel transistors on this panel, there would be great merit with respect to production cost, reliability, and the like, of the liquid crystal panel. At present, however, the crystallinity of the silicon film that forms an active layer of transistors is poor and, therefore, the performance of thin film transistors, represented by mobility, is low and it is difficult to fabricate an integrated circuit wherein high speed and high performance are required. As a method of improving the crystallinity of the silicon film for the purpose of implementing a thin film transistor with a high mobility, heat treatment is, in general, carried out by using a laser.
The relationships between the crystallinity of a silicon film and the mobility of a thin film transistor are described in the following. A silicon film gained through laser heat treatment is, in general, polycrystal. Crystal defects locate in crystal grain boundaries of the polycrystal silicon and they block the carrier movement in the active layer of the thin film transistors. Accordingly, the number of times that the carriers cross the crystal grain boundaries while moving through the active layer becomes less and the concentration of crystal defects becomes lower in order to enhance the mobility of the thin film transistors. The purpose of the laser heat treatment is to form a polycrystal silicon film of which the crystal grain diameters are large and wherein there are fewer crystal defects in the crystal grain boundaries.
FIGS. 18 to 20 are cross sectional views for describing a process for a polycrystal silicon film according to a prior art. First, referring to FIG. 18, a silicon oxide film 32 is formed on a glass substrate 31 by, for example, carrying out CVD (chemical vapor deposition) on a glass substrate. An amorphous silicon film 33 is formed on a silicon oxide film 32 by means of, for example, CVD.
Referring to FIG. 19, an excimer laser (KrF (wavelength: 248 nm)) irradiates an amorphous silicon film 33 in the direction shown by arrow 335. Thereby, the portion irradiated by the excimer laser melts. After that, as the temperature becomes lower, the melted silicon is crystallized so as to form a polycrystal silicon film 334.
Referring to FIG. 20, polycrystal silicon film 334 is patterned so that polycrystal silicon film 334 only remains in portions. Next, a silicon oxide film and a metal film (low resistance metal film such as Ta, Cr or Al) are formed on polycrystal silicon film 334. Gate insulating films 36a and 36b, as well as gate electrodes 37a and 37b, are formed by patterning the metal film and the silicon oxide film. Thereby, active regions 39a and 39b are formed. Next, source and drain regions are formed in a self-aligned manner by means of an ion doping method by using gate electrodes 37a and 37b as a mask. Thereby, the thin film transistors shown in FIG. 20 are completed.
According to a conventional method, as shown in FIG. 19, an amorphous silicon film is polycrystallized by using an excimer laser and, therefore, the mobility of the carriers is low in the transistors formed on the polycrystal silicon film. As a result, a high speed of operation of the transistors is difficult so that it is difficult to achieve a high response of the liquid crystal display device.
In addition, Reference 1 (T. Ogawa, et al., “Thin Film Transistors of Polysilicon Recrystallized by the Second Harmonics of a Q-Switched Nd: YAG Laser” EuroDisplay '99 The 19th International Display Research Conference Late-news papers Sep. 6-9, 1999 Berlin, Germany) discloses an amorphous silicon film made polycrystalline by using, for example, the second harmonic of an Nd: YAG laser as a laser light and a thin film transistor formed by using this polycrystalline film wherein the mobility is increased. However, since the output of the second harmonic of the YAG laser is small, an amorphous silicon film having only a small area can be made polycrystalline. Therefore, the manufacturing of a polycrystalline silicon film for manufacturing a liquid crystal display having a large area is difficult.
Therefore, this invention is provided in order to solve the above described problems.
One purpose of this invention is to provide a process and a unit for manufacturing a polycrystal silicon film that is suitable in the fabrication of a thin film transistor of high performance and that has a large area.
In addition, another purpose of this invention is to provide a thin film transistor of high performance and a process for the same.